A conventional flywheel is disclosed in AUTOMOBILE TECHNIQUE No. 12, pages 1428 to 1432 published by the Automobile Technical Association Corporation of Japan in 1985 for example. This flywheel is called a flywheel with a torsional damper and comprises a member on the side of an engine, a member on the side of a transmission, a compression spring, a hysteresis mechanism, a bearing for smoothly operating a damping member, and a limit torque mechanism for supressing an excessive torque generated when the flywheel is actuated.
The compression spring is flexed in accordance with the amount of an output torque of the engine, and in this state, a variable torque is absorbed by the compression spring and the hysteresis mechanism so that the member on the transmission side is rotated by a rotational amount damped and varied with respect to the member on the engine side, thereby transmitting an averaged torque to a drive system.
However, in the structure mentioned above, when the spring constant of the compression spring is increased, the variable torque can be absorbed without completely flexing the compression spring even when a large torque is transmitted, but the variable torque cannot be absorbed when a small torque is transmitted, e.g., when a vehicle is stopped in an idling state. When the spring constant of the compression spring is decreased, the variable torque can be absorbed when a small torque is transmitted, but when a large torque is transmitted, the compression spring is completely flexed so that the variable torque cannot be absorbed.
To solve these problems, a new system has been proposed in which a viscous coupling is disposed between a member on the engine side and a member on the transmission side so that the variable torque can be absorbed when relatively large and small torques are transmitted.
In the flywheel using such a viscous coupling, resistance plates are alternately engaged with the member on the engine side and the member on the transmission side, respectively, and are disposed within a sealed chamber formed between both members and filled with a viscous fluid such as silicon oil. Accordingly, when the resistance plates are rotated by the rotation of the member on the engine side, the connected to the transmission side member resistance plates are rotated through the viscous resistance of the viscous fluid, thereby transmitting the torque to member on the transmission side. The variable torque on the engine side is damped by the viscous fluid between the resistance plates so that the above-mentioned operation can be performed, whereby the variable torque can be absorbed when both large and small torques are transmitted. However, when the transmission torque capacity of the viscous coupling is set to be a value at which the variable torque can be absorbed even when a small torque is transmitted, the difference in the number of rotations between the member on the engine side and the member on the transmission side is increased when a large torque is transmitted, thereby reducing the torque transmission efficiency and increasing the fuel cost and further reducing the performance of the viscous coupling by wear of the resistance plates, etc.
In general, in a power transmission system disposed in a vehicle and provided with a flywheel rotated by source of an engine or the like, a vibration is generated in the power transmission system on an output side including an output shaft, the chassis, etc., when rapid variations in torque and the number of rotations of an input shaft are directly transmitted to the output shaft, thereby making it uncomfortable to ride in the vehicle.
To solve such problems, another type of conventional flywheel has been proposed examples of which can be found in Japanese Laid-Open Patents 57-204320 and 57-204348.
In this flywheel, a sealed chamber filled with a viscous fluid is disposed between an engine side member and a transmission side member. A plurality of resistance plates alternately combined with each other are disposed within the sealed chamber and are movable in the axial direction and are respectively engaged with the engine side member and the transmission side member, thereby constituting a viscous coupling. Further, an engaging device is disposed to vary the distance between the resistance plates by a biasing member biased in accordance with the increase or decrease of the number of rotations thereof. Thus, the torque transmission efficiency by the viscous resistance of the viscous fluid is made large, and a vibration caused by rapid variations in the number of rotations and torque is prevented from being generated in the case of low rotational speeds in which the generation of the vibrations is especially a problem. A sufficient torque can be obtained in the case of high rotational speeds.
Further, such a flywheel has a joint of a viscous fluid therein for relaxing rapid variations in the number of rotations of an input shaft and drive torque, thereby preventing vibration from being generated in a power transmission system.
In such a flywheel, rotary plates rotated by the joint the viscous fluid joint approach each other by force imparted from a movable member moved by a centrifugal force as the number of rotations of the flywheel is increased, thereby increasing the transmitted torque. Accordingly, in the case of low rotational speed of the flywheel in which the generation of vibration is especially a problem, the generation of vibration in the power transmission system is greatly reduced with respect to the rapid variations in the number of rotations of the flywheel and torque. In the case of high rotational speed of the flywheel, power can be transmitted at a high efficient rate.
However, in the conventional flywheel mentioned above, the transmitted torque is changed by the variation of the number of rotations of the flywheel, so sufficient power cannot be obtained when a large torque is needed at a low rotational speed in running states of a vehicle on a road having a bad condition, a steeply inclined road, etc.
Moreover, in such a flywheel, when the transmission torque capacity of the viscous coupling is set to be a value for enabling the variable torque to be absorbed even when a small torque is transmitted, the number of differential rotations between the engine side member and the transmission side member is increased when a large torque is transmitted, thereby increasing the fuel cost by the viscous differential rotation at the running time of the vehicle.